The overall objective of this proposal is to study the biological processes regulating the growth and regression of 7,12-dimethylbenz(a) anthracene (DMBA)-induced mammary tumors in rats. Most DMBA-induced mammary tumors are hormone-dependent and their growth or regression is regulated by the host's endocrine status. This proposal is based on the principle that any tissue growth is a result of a net gain of synthetic processes over tissue degradation. Thus, tissue synthesis and degradation play equally important roles in tumor growth. Understanding these processes could lead to the development of means to control malignant growth. The following subjects are the areas of our interest for the period proposed: (1) Differential and synergistic roles of estrogen and prolactin in protein and DNA synthesis in DMBA-induced mammary tumors. (2) Interrelationships between synthesis and degradative processes in growing and regressing tumors. (3) Whether or not tumor regression involves processes of active synthesis.